Abstract
Molecular hydrogen (H2) is the second most abundant trace gas in the atmosphere after methane1 (CH4). In the troposphere, the D/H ratio of H2 is enriched by 120‰ relative to the world's oceans2,3,4. This cannot be explained by the sources of H2 for which the D/H ratio has been measured to date (for example, fossil fuels and biomass burning)5,6. But the isotopic composition of H2 from its single largest source—the photochemical oxidation of methane—has yet to be determined. Here we show that the D/H ratio of stratospheric H2 develops enrichments greater than 440‰, the most extreme D/H enrichment observed in a terrestrial material. We estimate the D/H ratio of H2 produced from CH4 in the stratosphere, where production is isolated from the influences of non-photochemical sources and sinks, showing that the chain of reactions producing H2 from CH4 concentrates D in the product H2. This enrichment, which we estimate is similar on a global average in the troposphere, contributes substantially to the D/H ratio of tropospheric H2.
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Acknowledgements
We thank N. Kitchen for assistance in the laboratory. This study was supported in part by the NSF (T.A.R., K.A.B. and S.T.), the Davidow Fund and General Motors Corp. (J.M.E), the David and Lucile Packard Foundation (K.A.B.) and the NASA Upper Atmosphere Research Program (P.O.W., K.A.B, E.A, S.S. and S.D.). The National Centre for Atmospheric Research is operated by the University Corporation for Atmospheric Research under the sponsorship of the NSF. T.R. is the recipient of a Frederick Reines postdoctoral fellowship in experimental sciences awarded by Los Alamos National Laboratory.
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Rahn, T., Eiler, J., Boering, K. et al. Extreme deuterium enrichment in stratospheric hydrogen and the global atmospheric budget of H2. Nature 424, 918–921 (2003). https://doi.org/10.1038/nature01917
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DOI: https://doi.org/10.1038/nature01917
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